After another 24 h, both rigs keep seeming to be stable. I suspect they are stable.
How can this be? I was convinced that the i5 760 was just an i5 750 with an increased multiplier (20 -> 21). But in my case, the Gigabyte motherboard - when used with 2x4 GB RAM - is obviously not compatible with my i5 760, but fully compatible with an i5 750...